Just curious, is there a certain time frame you go on, or just how dark the line is and stops at?

We BD last night.. (im not currently TTC).. and only just realised this morning i am semi near O.. so i did a OPK test this morning to see how close it was. Normally i get O pains and havent had any EW cm yet (though both can appear within hours of each other). The test line was light to start with but after 10 mins or so was just as dark as the control line.

So do you go off how dark it gets all together or is it just after the first few minutes?